-How Did Islam Spread here?
In opposition to the new states dominated by Arabic rulers, the people of the desert(Berbers) formed their own states in areas such as Fez, and Sijilmasa, old trading cities.By the 11th century, under pressure from Muslim invaders, a strong puritanical reform movement developed, including such groups as the Almohadis, and Almoravids , who launched a Jihad(Holy War) into Spain. This was the basis of Islamic spread into Sub-Saharan Africa.
The beginning of trans-Saharan trade, made possible by the domestication of the camel, profoundly influenced the world of sub-Saharan Africa. Gold, salt and slaves began to make their way across the desert. With them came Islam. Because Islam does not separate religious authority from political authority, it was most appealing to tribal leaders because it strengthened the African concept of kingship. Kings who converted had more power and authority at their disposal. Several Muslim empires would emerge as a result.(http://www.historyhaven.com/APWH/unit2/Organization%20and%20Impact%20of%20Islam.htm)
Elites were the earliest converts to Islam in sub-Saharan countries, long before the population’s vast majority,( farmers and herders), got in touch with the religion. It was most likely from the elites’ interaction with merchants who came across the Sahara desert to the capital cities.Elites had more obvious contact with the merchants than the rest of the population, and thus more exposure to the spread of Islamic ideas carried by said merchants.( http://muslimvoices.org/islam-spread-subsaharan-africa/ )

Examples of Syncretism in Sub-Saharan Africa:
The common people did not practice Islam in as pure a form as did the kings and other people of influence. Most people combined it with their established beliefs of ancestor worship and fetishes(idol and symbol worship). Nor did it greatly affect gender roles. “Women in sub-Saharan Africa possessed more opportunities than did women in other parts of the world. Even the arrival of Islam did not substantially worsen the condition of women in sub-Saharan Africa.”

Growth of Trade due to Islam:
For reasons described above, Islam dramatically increased trade in Sub-Saharan Africa. It also increased the slave trade. Muslims considered the enslavement of unbelievers as a step toward their conversion. Also, in Islamic law persons born to slave parents were not automatically slaves, as in the American South. This meant that there was a constant demand for slaves because each generation of slaves had to be purchased anew. Moreover, “private ownership of land was not an established institution in Sub-Saharan Africa, a fact that made the possession of slaves an important barometer of personal wealth. As many as ten million African slaves were shipped north as part of the trans-Saharan slave trade between 750 and 1500 C.E.”
Urbanization due to Islam:
Cities existed in other parts of Africa during the precolonial period. In ETHIOPIA, the city of AKSUM was an important economic, political, and religious center. Along the Indian Ocean coast in East Africa, urban centers sprang up at ports that served seafaring traders from the Arabian and Indian peninsulas. Kilwa, with magnificent mosques, palaces, and baths, was the most splendid East African city in the 1400s.( http://www.jiffynotes.com/a_study_guides/book_notes/aes_01/aes_01_00087.html )

Islamic Architecture:
Of the buildings of the continent south of the Sahara, the ruins of Great Zimbabwe are perhaps the best known. This complex of stone enclosures, particularly those popularly termed the elliptical building and the acropolis, was built on sites established as early as the 3rd century CE. The first Shona phase of building was probably begun six centuries later and continued until the 15th century, when, under the Mwene Matapa, or “Ravager of the Lands,” Zimbabwe reached its peak.
Photograph:Aerial view of the ruins of Great Zimbabwe.
Photograph:Aerial view of the ruins of Great Zimbabwe.

(Aerial view of the ruins of Great Zimbabwe) http://www.britannica.com/blackhistory/article-9384737
The architectural forms of Great Zimbabwe, however, are atypical of many African architectural styles. The site has a massive defensive wall and, included in the elliptical building, a conical tower of unknown purpose. It is also monumental in scale, having functioned as a royal citadel, and it has become a national symbol. While some of these features can be found in other examples of African building, they are rare, and the emphasis on Zimbabwe has overshadowed the great diversity of materials, forms, purposes, and uses characteristic of architecture elsewhere in Africa.

Urbanization due to Islam:
Afro-Islamic architecture flourished best in the Saharan empires. Although mosques were also constructed along the Swahili coast (mainly from coral), no striking new aesthetic evolved there, such that Swahili buildings are similar to those of the core Islamic world. At any rate, most Swahili cities were destroyed by the Portuguese during the Early Modern age; the best-preserved settlement is Lamu, a Kenyan town that was once a booming city-state (see mosque in Lamu external image ext.gif)30.A distinctive style of mud brick architecture developed in the Saharan empires, featuring thick sloping walls, flat roofs, and conical spires30. This "Saharan style" of architecture is unmistakable for its grids of structural wooden beams that extend far beyond a building's exterior. The foremost work of the style is the Great Mosque of Djenné, the world's largest mud brick building.( http://essentialhumanities.net/artw14.html )